The basic functionality of a Global Positioning System (GPS) receiver is to calculate the latitude, longitude and altitude of the GPS receiver's location (i.e. the co-ordinates of the receiver) upon receiving a number of GPS signals from a network of GPS satellites that orbit the earth. The calculation of the co-ordinates of the GPS receiver typically begins by comparing the timing associated with a select number of received GPS signals. After the initial comparison of the received GPS signals, values for timing corrections associated with the select group of received GPS signals are established. The timing corrections are made in order to solve a three-dimensional geometric problem, which has as its solution the co-ordinates of the GPS receiver.
The received GPS signals are typically weak and thus easily interfered with by other radio transmissions in the same or adjacent frequency bands. Interference can be especially problematic when the GPS receiver is co-located with a communications device that includes a radio transmitter such as a cellular telephone. The transmitted signal from the co-located communication device can overload or saturate the GPS receiver front-end designed to receive weak GPS signals. In such a situation, no useful information can be extracted from the received GPS signals originating from the GPS satellites.
One known method of overcoming this problem is by filtering all of the received signals from the GPS antenna before down conversion of the respective transmission signal band by the GPS receiver front-end. Typically a low noise amplifier (LNA) is first used to amplify the signal before further filtering or mixing to another frequency. The result of adopting this approach is that the loss of all signal energies in the filter reduces the sensitivity of the GPS receiver permanently, irrespective of whether or not the co-located communications device is transmitting. This is an undesirable result as the GPS signals received from the satellites are weak and reducing the sensitivity of the GPS receiver further reduces the operability of the system. Additionally, the filter would also occupy space, and add cost to the unit.
Another approach in dealing with the effects of co-located communications device is overcome by the use of a high linearity LNA. This ensures that the LNA is capable of amplifying the GPS signal despite the presence of a large interfering locally generated transmission. The disadvantage of this solution is that such an LNA would consume additional power, which is not acceptable in a portable battery powered device such as a cellular telephone. A filter following the LNA would also be required to provide sufficient rejection of the interfering signal to prevent overload of the next stage of the receiver, typically a mixer. These additional performance requirements increase the size, power consumption and cost of the filter and make implementing a highly integrated receiver design without the additional filter difficult.
There are several prior art schemes that further deal with minimizing the effects of a cellular transceiver and antenna on the GPS receiver operation by blanking GPS signals during operation or transmission of radio receiver signals. These systems are implemented by sending a blanking signal to the GPS receiver upon detecting the presence of a jamming signal outside of the GPS receiver. This does however increase the complexity of system integration as an appropriate dynamic blanking signal needs to be provided.
It is therefore an object of the invention to provide a relatively efficient method for minimizing the effect of interfering transmission without blanking the automatic gain control (AGC) or correlator of the GPS receiver or deactivating the GPS receiver based upon a detected interference.